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Six months ago the brightest gamma-ray burst ever seen from Earth was picked up by the SWIFT telescope, resulting in the rapid scrambling of telescopes all over the world to monitor the fading light, and giving astronomers the most detailed portrait yet of a stellar explosion.

In today’s issue of the journal Nature, Judith Racusin, a graduate student from Penn State University, leads a paper with 92 co-authors that report observations across the electromagnetic spectrum of the explosion. Another paper, led by Joshua Bloom of the University of California, Berkeley was also accepted for publication in The Astrophysical Journal this week.

Gamma-ray bursts (GRBs) are the Universe's most luminous explosions, resulting at the end of a massive star’s life as it runs out of nuclear fuel, collapsing in on itself to create a black hole or neutron star that, through processes not yet fully understood, drive powerful gas jets outward into space. These jets punch through the collapsing star, striking gas previously shed by the star and heating it up, generating bright afterglows that can later be picked up by telescopes and sometimes even with the naked eye.

One such gamma-ray burst, GRB 080319B, was detected by NASA’s SWIFT satellite on March 19 this year, although no one spotted it with their naked eye. It was thought to be the result of a massive star exploding 7.5 billion years ago, and it sent a jet of intense light on a direct collision course for Earth at 99.99995 percent the speed of light.

"This was the brightest optical and infrared event that mankind has ever recorded," says Bloom. Bloom's colleague Alex Filippenko calculated that if the supernova were instead located about 6,000 light years from Earth, the GRB would have appeared as bright as the Sun. Even at its peak, the burst was about 200 million times brighter than the entire galaxy in which it occurred. But it was unlikely that anyone actually saw the flash, because it lasted only a few tens of seconds and appeared on a night with a bright Moon. Astronomers also now know that it reached a magnitude of around 5.6, close to the limits of human vision.

In the seconds after it first pointed at GRB 080319B, the infrared telescope PAIRITEL was blinded by the brilliant explosion (a). As the burst began to fade, PAIRITEL was able to track the light from the explosion for many hours (b and c). Image: PAIRITEL.

Judith Racusin was making observations with both SWIFT's X-Ray Telescope and its UltraViolet/Optical Telescope of a different burst when it captured the explosion of GRB 080319B less than 10 degrees away, which blinded the telescope for a few seconds. "Within minutes, as reports from other observers arrived, it was clear this explosion was an especially extraordinary event," she said.

Many ordinary optical telescopes on the ground, such as the infrared telescope PAIRITEL, met a similar fate – saturated by the initial burst, and only able to begin study several minutes later. Other telescopes, including Pi of the Sky, REM/ TORTORA and RAPTOR, allowed scientists to study for the first time exactly what a gamma-ray burst looks like in optical light all the way from beginning to end.

Along with SWIFT, the Russian KONUS detector on NASA's Wind satellite also monitored the evolution of the gamma rays, providing a wide view of their spectral structure. Thanks to the resulting global effort in analysing the GRB, astronomers now know that the jet contained an ultra-fast component just 0.4 of a degree across, embedded within a slightly less energetic jet about 20 times wider. The broad component is more typical of what SWIFT has seen in other GRBs.

Click for animation showing how astronomers think GRB 080319B erupted. A narrow, ultrafast jet first punches through the star and is followed by a wider, less energetic jet. Image: NASA/Swift/Cruz deWilde.

"We normally detect only the wide jet of a GRB as the inner jet is very narrow, equivalent to not much more than 1/100th the angular size of the full Moon,” says Dr Paul O'Brien of the University of Leicester, part of the SWIFT team. “It seems that to see a very bright GRB the narrow jet has to be pointing precisely at the Earth. We would expect that to happen only about once per decade. On March 19th, we got lucky."

"When more of these events are detected, we will open up the possibility of studying the infant Universe with this new tool,” adds Bloom. The fact that an event was visible to the naked eye at a distance of 7.5 billion light years implies that if similar explosions occur even further away, for example at a distance from Earth of 13 billion light years or more, that is, right back at the start of the Universe’s history, current and future telescopes would be able to study them in great detail, giving astronomers an unrivaled look at the earliest era in cosmic history.

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